Echo planar imaging (EPI) is a magnetic resonance (MR) imaging technique capable of acquiring an entire MR image in short amount of time compared to other conventional MR techniques. While conventional MR techniques progressively generate an image from a series of sequentially acquired readout lines, EPI obtains multiple lines of spatially-encoded data of an image after a single excitation, either in a gradient echo or spin echo pulse sequence. This excitation is commonly referred to as a “shot” by those skilled in the art. When EPI is used to acquire all lines of an image after one shot, it is commonly referred to as a “single-shot EPI” acquisition.
Conventional systems apply accelerated parallel imaging technique to EPI acquisitions to reduce a number of phase encoding steps and therefore to reduce a total readout time and echo spacing. In turn, this decreases image blur and geometric distortion artifacts caused by off-resonance effects. A GRAPPA (Generalized Autocalibrating Partially Parallel Acquisition) accelerated parallel imaging method is used in conventional systems for image reconstruction due to its use of reference autocalibration data to facilitate image reconstruction, rather than use of explicit coil sensitivity profiles. Although GRAPPA is somewhat insensitive to image contrast contained in the reference autocalibration data, typically the acquisition of the reference autocalibration data is performed so as to match echo spacing of image data so that geometric distortions present in the image data are also present in the autocalibration data.
In order to match the echo spacings between the image data and the reference data, conventional imaging systems employ a multi-shot or segmented EPI acquisition for k-space reference line acquisition. For time efficiency in multi-slice acquisitions, segmented reference lines are acquired in a consecutive-slice manner in which a first segment is acquired across slices in a collection of acquired slices, followed by a second segment and so on. This causes a time interval between the adjacent segments within a given slice to be a function of the number of slices and the repetition time, TR. This interval can be, for example, between 2 to 10 seconds for standard imaging protocols. A difficulty of this method is that a misalignment between adjacent segments of the reference line acquisition leads to pronounced image artifacts in GRAPPA reconstructed images. Misalignment occurs if, for example, a patient moves during an interval between segments. Furthermore, because changes in breathing or respiration affect the EPI data, if the adjacent segments of the reference line acquisition are acquired during different phases of the respiratory or cardiac cycles this introduces errors into the GRAPPA training that propagate into artifacts in the final image reconstruction.